Process for recovering separated olefins and diolefins



2,705,698 Patented Apr. 5, 1955 PROCESS FOR RECOVERING SEPARATED OLEFINS AND DIOLEFINS Glen P. Hamner, Baton Rouge, La., assignor to Esso Research and Engineering Company, a corporation of Delaware Application May 21, 1952, Serial No. 289,111 3 Claims. (Cl. 202-69) The present invention relates to the separation of C2 to C olefins and diolefins from a complex hydrocarbon stream containing these components.

A large variety of reactive olefinic hydrocarbons are formed when hydrocarbons boiling in the naphtha through gas oil range are subjected to high temperature vapor phase cracking, preferably in the presence of steam. The valuable C2-C5 olefinic cracked products include ethylene, propylene, butylenes, butadiene, isoprene, cyclopentadione, and piperylene.

Cracking processes for obtaining the olefinic cracked products are known and there are processes for segregathitherto developed mainly for separating the C1 to C4 olefins, as shown in U. S. Patent 2,348,659 of B. I. Smith et al.

The newer developments with which the present invention is concerned have to do with the additional recovery of the C5 and Cs cliolcfins which have a tendency to undergo more or less dimerization, codimerization, and polymerization at moderately elevated temperatures, such as met in recovery systems.

It has been found that certain steps deviating from the normal procedure of recovering only C1 to C4. olefins have to be taken for eflicient separation of more of the C5 diolefins. One of these steps deals with selection of a portion of a water-wet feed stream for a dehydrating treatment which is important in the recovery of the low molecular weight hydrocarbons, e. g. ethylene and propylene. In conjunction with a high recovery of the ethylene and propylene components, steps are provided for separating a maximum amount of these components from the higher boiling materials with the smallest loss of the diolefins by polymerization.

Preferred steps will be described with reference to the flow diagram in the drawing.

A vapor-phase cracked hydrocarbon stream containing largely olefins and diolefins in the C2 to C5 range from line 1 is cooled in cooler 1C and received in the feed accumulator 2. In the accumulator 2 the conditions are maintained for separating substantially all C1 and C2 hydrocarbons in the gaseous phase, but under these conditions an appreciable amount of C; and'C4 hydrocarbons are withdrawn in thegaseous stream by line 3. Suitable temperatures and pressures in the separating ,zone or accumulator 2 are: 25 to 30 C. and 170 p. s. i. g.

The residual liquid continuously withdrawn from the accumulator 2 by line 4 essentially contains substantial amounts of C and C4 hydrocarbons with the higher hydrocarbons. This residual stream is passed by line 4 into a mildly heated stripping zone 5 wherein conditions are maintained for stripping out substantially all C3 hydrocarbons. The C3 hydrocarbons with an appreciable amount of C4 hydrocarbons unavoidably stripped therewith are passed from the upper part of the stripping zone 5 through line 6 and can be joined with the initial gaseous stream removed by line 3 from the accumulator 2. Liquid condensate from line 6 is returned as reflux to zone 5 through line 7. A pressure of about 170 to 180 p. s. i. g. is maintained in the stripping zone 5 with a temperature of 60 to 70 C. at the top and temperatures of .130 to 150 C. at the bottom for eifecting substantially complete removal of C3 hydrocarbons. A reboiling means 8 may be provided at the bottom of the stripping column 5. Thus, satisfactory removal of including the C3 hydrocarbons is effected in zone 5 without excessive temperature effects on the remaining higher boiling reactive diolefins.

the lower hydrocarbons The residual C4 and higher hydrocarbons are withdrawn from the bottom of zone 5 by line 9 to be processed as will be further described.

The gas and vapor streams in line 3 and 6 are advantageously combined to be processed in a dehydration unit. As these vapor streams generally contain substantial to subject these streams to a chemical treatment, e. g., scrubbing with aqueous caustic solution. For this purpose the combined streams from lines and are passed is supplied to the top of the scrubber 10 the spent caustic solution, containing the impurities, In the scrubber, temperatures of about to C. may be maintained and a pressure of about 1.80 to 215 p. s. i. g. in order to keep the hydrocarbons in phase. The desulfurized hydrocarbon vapors from the top of the scrubber 10 by line: 13

through a cooler 14 to lower their temperature to about 10 to 20 C. Such cooling can be economically carried out by heat exchange with water and ammonia refrigeration. The cool vapors are passed from the cooler through line 15 into a knock-out drum 16 maintained at l0-15 C. and 150-460 p. s. i. g.

The knock-out drum 16 serves an important function of separating condensate formed in the cooler 14 from remaining gaseous hydrocarbons. With the cooler 14 and knock-out drum 16 or similar means, the hydrocarbons remaining in the gas phase can be kept practically free of C5 and higher hydrocarbons. It has been found removed sulfur desirable to subject the C1 and C2 hydrocarbons with the smallest amount or" C4 and C5 hydrocarbons present to dehydration required in recovering ethylene. The gaseous stream containing the ethylene with C3 hydrocarbons, a

small amount of C4 hydrocarbons and a slight amount of C5 hydrocarbons, is withdrawn from drum 16 by line 17 to be passed to the dehydrator 18.

A practical dehydration uses a material such as alumina or similar desiccating agent for adsorbing or absorbing water from the gas. The moisture-free gas is passed by line 19 into a compressor 20 and cooling means 21 for partial liquefaction, then is fed from line 22 into the fractionating tower 23.

Fractionating tower 23 is-operated under pressures of about 550 to 570 p. s. i. g. with a top temperature of about 0 to -25 C. and a bottoms temperature of about 50 to C. to vaporize and expel methane with some ethylene. The thus dilute ethylene is withdrawn overhead through line 24. Refiuxmg means 25 is provided at the top. The bottoms liquid of mainly ethylene and propylene is withdrawn by line 26 to the next tower 27.

Tower 27 is operated to distill a concentrated ethylene:

fraction. It is operated under pressures of about 410 to 460 p. s. i. g. with a top temperature of about 0 to -5 C. and a bottom temperature of about to C. The concentrated ethylene distillate is withdrawn overis withdrawn from the bottom by line 30. 1

It is to be noted that in separating the lowest boiling hydrocarbons, e. g. methane, ethane, and ethylene, temperatures below the freezing point of water are required in the upper part of the separating columns, with gives the reason for having the hydrocarbons substantially free of water. Liquefied proplyene may be used as a refrigerant for cooling the upper parts of the columns.

The propylene concentrate withdrawn as bottoms from bottoms product is passed by line 35 into tower 36 open nts of sulfur as well as water, it is found desirable is Withdrawn from the scrubber by line 12.

the vapor withdrawn C. The propylene distillate is taken overhead oils boiling in the range of 100 ated under conditions for separating C4 hydrocarbons as the overhead product.

Column 36 is operated under a pressure of 80 to 90 p. s. i. g. with a top temperature of about 55 to 60 C. and a bottom temperature of about 110 to 115 C.

Under these conditions the bottoms are left substantially free of C4. hydrocarbons and will comprise mainly C5 olefins and diolefins, which is a desired composition for further processing to separate cyclopentadiene, isoprene, and piperylene. The cyclopentadiene may be recovered by soaking the C5 concentrate at temperatures ofthe order of 80 to 115 C. (180 to 240 P.) which is suitable for selective dimerization of the cyclopentadiene in periods ranging from about 4 to hours. Column 36 is provided with overhead take-off line 38 and refiuxing means 39 The present invention is concerned with recovering a satisfactory C5 concentrate mixture of olefins and diolefins but not with the further processing of'this concentrate since such processing has been described in the art.

' Included in the C5 concentrate bottoms from tower 36 will be the heavy ends of the condensate fraction withdrawn from the knock-out drum 16 by line 40. This condensate from the knocloout drum is purposely made to bypass the dehydrator and the highly refrigerated columns 23 and 27. This condensate from the knockout drum will contain some Cs and C5 hydrocarbons, but mainly C4 components. A large number of benefits are obtained by sending this material directly to the propylene'recovery tower 32. Another stream which may contain appreciable amounts of C4 and C5 hydrocabons is. the overhead vapor stream removed by line 41 from the top or upper part of column 42. Column 42 receives the bottoms from the stripping tower 5. Column 42 may be operated to maintain relatively moderate temperatures in the upper part, c. g. 38 to 70 C. for removing C4 and C5 components under a pressure of 100 s'fi. J The bottom. temperature of column 42 is about 150 to 160 C. Another. side stream containing higher C5 components mixed with C6 components may be withdrawn from column 42 by line 43. A bottoms fraction of Cs+ components is withdrawn by line 44. Some of. the bottoms is recycled through reboiler 45.

The C4 and C5 hydrocarbons removed overhead from column 42 maybe passed into column 36 for separating the C4 component and collecting the additional C5 components in the bottoms thereof.

EXAMPLE The following example illustrates typical conditions and results of the process:

Gas streams containing C1 to Cs+ olefins and dipetroleum distillate to 480 C. in vapor phase mixed with 60 to 80 mole per cent steam to temperatures of 680 to 700 C. under pressures of 13 to 20 p. s. i. g. Cracking products leaving the cracking zone were quenched to about 230 C. and fractionated to separate the gas streams. The gas streams were compressed from about 4 p. s. i. g. to 580 p. s. i. g. and cooled to about 40 C. to obtain the charge to the accumulator 2. This accumulator acted as a gas-liquid separating zone under conditions that substantially all the ethylene remained in the gas phase.

Analyticalstudies showed that when the ethylenecontaining water-wet gas being passed through the caustic scrubber and then into the dehydrator contained considerable quantities of CrCf) hydrocarbons including diolefins, suchfeeds carried excessive amounts of moisture into the dehydrator, thus'shortening the life of the desiccant. The diolefins in the C4 and C5 range form polymers, which coat the surface area of the desiccant. This reduces the efiiciency and capacity of the drier sec- The problem arose whether it was feasible to condense out part of the C4 hydrocarbons and higher hydrocarbon components from the ethylene-bearing gas stream and to divert this condensate away from the ethylene separation tower without upsetting the recovery of ethylene.

The partial condensate from the gas stream containing part'of the C4 with the C5 and higher components was bypassed from drier and ethylene removal sections and sent directly olefins were produced by heating knock-out drum 16 around the mixture prior to to the propylene seperation column 32 as explained with reference to the drawing. It was determined that this reduction of the material sent through the dehydrator section to the ethylene separation zone gave a better dehydration operation and improved ethylene fraction. In addition, an improved recovery of C5 obtained.

Table STREAM ANALYSIS p. d S Cri ndensatc r d 2n tage rior to ce to De- Strwm g figg g Condensate Dehydrahydrators (Line 4) tors (Linc (Line 17) 40) Hz 0. 5 1 0.1 14. 9 0. 9 1. 5 25. 8 0. 4 0.6 11. i 8. 4 10. S 27. 0 0.8 1.6 2. 7 7. 4 11.1 4.1 18. 4 27. 7 l0. 7 0.7 1.0 0. 4 2. 3 3. 2 0. 9 4. 8 0. 5 4. 5 0.1 17. 1 23. 7 l. 7 0.3 0. 1 trace 34. 0 trace trace 100.0 100.0 i 1000 The elimination of some of the intermediate treatment on the C4 and C5 diolefins accounts for increased recovery of these diolefins, because by exposing the reactive C4 and C5 diolefins to temperatures above 80 C. and the vapor phase contact with a desiccant, particularly under elevated temperatures and pressures, these substances tended to undergo undesired interpolymerization. By the interpolymerization these diolefins become converted to substances which are high boiling and not useful for recovery of the desired C4 diolefins or the C5 diolefins and their dimers.

What is claimed is:

1. In a process of separating C2 to C5 olefins and diolefins from a water-wet gaseous mixture thereof, the improvement which comprises cooling said gaseous mixture to condense therefrom C5 components with part of the C3 and C4 components as an initial partial condensate to leave a remaining gas stream containing C2 with uncondensed C3 and C4 components, cooling said remaining gas stream to further condense out a substantial amount of the C3 and Cr components as anintermediate partial condensate, then drying the remaining gas stream containing C2 to C4 components relatively enriched in C2 components to remove water vapor therefrom, fractionating from the resulting dry C2 to C4 components an ethylene distillate to leave a liquid residue containing C3 and C4 components, passing said liquid residue into a propylene distillation zone, directly passing said intermediate partial condensate of C3 and C4 components into said propylene distillation zone, distilling overhead C3 components in said propylene distillation zone and withdrawing C4 components as a bottoms product from said propylene distillation zone.

2. In a process of separating olefins and diolefins from a water-wet gaseous mixture containing C1 through C5 and higher olefinic hydrocarbons, the improvement which comprises condensing from said mixture part of the C4 hydrocarbons with the C5 and higher hydrocarbons, desulfurizing the remaining gaseous mixture containing C1 to C4 hydrocarbons in a desulfurizing zone, cooling the desulfurized gaseous mixture to condense therefrom a condensate containing part of the C3 hydrocarbons with most of the C4 hydrocarbons, contacting the remaining desulfurized gaseous hydrocarbon mixture through a dehydrating zone, separating methane and ethylene fractions from the dried and desulfurized hydrocarbon mixture, then forming a combined stream of the residual Cs and C4 hydrocarbons which passed through the dehydrating zone with the condensate containing the C3 hydrocarbons and C4 hydrocarbons removed from the desulfurized gaseous the drying thereof, and separating the C5 and C4 hydrocarbons from said combined stream.

3. In a process of separating olefins and diolefins from a water-wet gaseous mixture of C1 through C5 hydrocarbons, the' steps which comprise condensing from said mixture the C5 components with part of the C3 and C4 components, further condensing C3 and C4 components diolefins was from the water-wet gaseous mixture to leave the remaincomponents from said combined liquids by fractional ing gaseous mixture substantially free of C5 components distillation. and with a decrease content of C4 diolefins, drying the gaseous mixture wlich is free of C5 components and of References Cited in the file of this patent decreased C4 diole 11 content, separating C3 and C4 corn- 5 ponents as liquids from the thus dried gaseous mixture, UNITED STATES PATENTS combining the C3 and C4 components separated as liquids 2,324,955 Rupp et a1 July 20, 1943 with the C3 and C4 components condensed as liquid from 2,368,497 Shipley et al Jan. 30, 1945 the water-wet gaseous mixture, and separating the C3 2,514,294 Rupp July 4, 1950 

1. IN A PROCESS OF SEPARATING C2 TO C5 OLEFINS AND DIOLEFINS FROM A WATER-WET GASEOUS MIXTURE THEREOF, THE IMPROVEMENT WHICH COMPRISES COOLING SAID GASEOUS MIXTURE TO CONDENSE THEREFROM C5 COMPONENTS WITH PART OF THE C3 AND C4 COMPONENTS AS AN INITIAL PARTIAL CONDENSATE TO LEAVE A REMAINING GAS STREAM CONTAINING C2 WITH UNCONDENSED C3 AND C4 COMPONENTS, COOLING SAID REMAINING GAS STREAM TO FURTHER CONDENSE OUT A SUBSTANTIAL AMOUNT OF THE C3 AND C4 COMPONENTS AS AN INTERMEDIATE PARTIAL CONDENSATE, THEN DRYING THE REMAINING GAS STREAM CONTAINING C2 TO C4 COMPONENTS RELATIVELY ENRICHED IN C2 COMPONENTS TO REMOVE WATER VAPOR THEREFROM, FRACTIONATING FROM THE RESULTING DRY C2 TO C4 COMPONENTS AN ETHYLENE DISTILLATE TO LEAVE A LIQUID RESIDUE CONTAINING C3 AND C4 COMPONENTS, PASSING SAID LIQUID RESIDUE INTO A PROPYLENE DISTILLATION ZONE, DIRECTLY PASSING SAID INTERMEDIATE PARTIAL CONDENSATE OF C3 AND C4 COMPONENTS INTO SAID PROPYLENE DISTILLATION ZONE, DISTILLING OVERHEAD C3 COMPONENTS IN SAID PROPYLENE DISTILLATION ZONE AND WITHDRAWING C4 COMPONENTS AS A BOTTOMS PRODUCT FROM SAID PROPYLENE DISTILLATION ZONE. 